How do rates of myocarditis and pericarditis compare between mRNA vaccines and the J&J vaccine long term?
Executive summary
Population and surveillance studies show myocarditis and pericarditis have been observed more often after mRNA COVID-19 vaccines than background levels—especially in males aged ~12–29 and most commonly after dose two—with reported peaks like ~27 cases per million doses in males 12–24 (FDA) and higher age/sex–specific estimates in some studies (eg, 50–147 per million in young males in BMJ synthesis) [1] [2]. Available sources do not directly provide long‑term, head‑to‑head comparisons of myocarditis/pericarditis incidence after mRNA vaccines versus the adenoviral‑vector Janssen (J&J) vaccine over multiple years; most analyses focus on short‑term risk windows (days to weeks) and age/sex strata [3] [4] [2].
1. Short‑term signal is concentrated and measurable
Multiple population and surveillance studies consistently found an elevated, but still rare, risk of myocarditis/pericarditis in the short interval after mRNA doses—especially after dose two and in younger males—measured in days to a few weeks post‑vaccination. Examples: the FDA’s updated label summarized an estimated ~8 cases per million doses across ages 6 months–64 years and ~27 per million in males 12–24 in the 1–7 day window for recent mRNA formulas [1]. Large cohort and passive‑surveillance analyses report higher age‑ and sex‑specific peaks—eg, reviews and syntheses put incidence ranges for males 12–29 anywhere from dozens to over 100 cases per million after dose two in some estimates [2] [5].
2. Magnitude depends on age, sex, dose number and mRNA product
Data show the myocarditis signal is not uniform: males and adolescents/young adults account for most cases; second doses carry higher risk than third doses in many reports; Moderna (mRNA‑1273) often shows higher rates than Pfizer (BNT162b2) in some datasets [5] [6] [2]. For instance, a CMAJ population study found higher cases per 100,000 doses in ages 12–17 and 18–29, and a higher observed‑to‑expected ratio after Moderna in males 18–29 [6].
3. Comparisons with J&J (adenoviral vector) vaccines: limited direct evidence in sources
Available sources provided to this review do not include a multi‑year, direct head‑to‑head long‑term comparison between mRNA vaccines and the Janssen (J&J) adenoviral‑vector vaccine specifically for myocarditis/pericarditis. A study in England compared adenovirus (ChAdOx1) and mRNA vaccines for myocarditis in short windows, but long‑term comparative incidence beyond the initial post‑dose risk windows is not reported in those excerpts [4]. Therefore, claims about long‑term comparative rates between mRNA and J&J are not documented in these provided reports: not found in current reporting.
4. Longer‑term outcomes and recovery: evidence points to mostly favorable short‑term course but limited long‑term data
Case series and follow‑up studies cited by industry and academia report that vaccine‑associated myocarditis has generally been treatable and patients often recover, with many studies focusing on cardiac MRI and clinical follow‑up; however, high‑quality, long‑duration outcome data remain limited in the sources supplied [7] [8]. Systematic reviews note very few long‑term outcome studies and classify long‑term evidence quality as low to very low in many subgroups [2].
5. SARS‑CoV‑2 infection still causes myocarditis at higher rates than vaccination in many analyses
At least one large study and reviews emphasize that myocarditis from COVID‑19 infection often exceeds post‑vaccine myocarditis risk, an important contextual point when weighing risks and benefits [4]. This comparison is noted in population‑level self‑controlled analyses though age‑stratified nuances remain.
6. Areas of disagreement, uncertainty and potential bias to watch
Surveillance systems (VAERS, passive reporting) and cohort studies differ in methods—leading to a range of incidence estimates; manufacturers’ and regulators’ summaries use different datasets and windows, producing different numerators and denominators [9] [1]. Nordic countries took a precautionary stance about Moderna in young people based on preliminary registry data, illustrating how jurisdictional policy can diverge when signals are uncertain [10]. Recent political claims about withheld data appear in reporting but do not change the underlying epidemiologic findings summarized by regulators and peer‑reviewed studies [11].
7. What a reader should take away now
Available sources show a reproducible short‑term myocarditis/pericarditis signal after mRNA vaccines concentrated in young males and after second doses, with absolute rates generally low (tens per million in peak groups) and regulatory labels updated to reflect that risk [1] [2]. The specific question of long‑term, head‑to‑head incidence comparing mRNA vaccines to J&J over years is not answered by the provided material—further direct comparative, long‑follow‑up studies would be required to resolve that gap [3] [4].
Limitations: this analysis uses only the supplied sources; it reports short‑term risk windows and surveillance results primarily—longer‑term comparative data for mRNA versus J&J are not present in these materials [3] [4].